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Understanding the Giant Seebeck Coefficient of MnO 2 Nanoparticles Costel Constantin James Madison University James Madison University, October 2012

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- Crystal structures and semiconductor properties. - Materials characterization methods. - Giant Seebeck Coefficient Observed in Manganese Oxide Nanostructures? Outline

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Crystal Structures WHAT KEEPS THE ATOMS TOGETHER INSIDE OF A CRYSTAL? unique arrangement of atoms in a crystal. composed of a unit cell, which is periodically repeated in three dimensions on a lattice. a = lattice constant

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Types of Crystal Structures

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Forces Between Atoms in a Crystal IONIC BONDS - electrostatic forces between two oppositely-charged ions, e.g. alkali halogenides METALLIC BONDS - electrostatic attraction between the metal atoms or ions and the FREE electrons, also called CONDUCTION electrons. e.g. Metals. COVALENT BONDS - sharing of pairs of electrons between atoms, e.g. Semiconductors, Organic Molecules; C, Si, InSb. VAN DER WAALS BONDS arises from the polarization of molecules into dipoles. e.g. Noble Gas crystals, H 2, O 2. HOW ABOUT THE ENERGY LEVELS IN A CRYSTAL?

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Electronic Band Structure in Solids Electrons live in ENERGY ORBITALS = ENERGY LEVELS. ENERGY LEVELS in a crystal, where ions bond, form ENERGY BANDS. HOW CAN WE UNDERSTAND THE DIFFERENCE BETWEEN METALS, INSULATORS, AND SEMICONDUCTORS IN TERMS OF ENERGY BANDS? E 1s 2s 2p 3s 3d 3p

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Metals, Insulators, Semiconductors WHAT IS THE MECHANISM FOR SOME MATERIALS TO CONDUCT ELECTRICITY? Define E F as the level below which all electrons fill up the states (little cups). METALS - Fermi energy level falls at the middle of the allowed band. INSULATORS and SEMICONDUCTORS - Fermi energy level falls at the middle of the forbidden gap. 1s 2s 2p 3s 3d 3p

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Conduction Bands, Valence Bands, and Band Gaps VALENCE BAND - created by the outer shell electrons, and most of the states (cups) are occupied by electrons. CONDUCTION BAND - free electrons coming from VB and able to conduct electricity. BAND GAP - the width of the forbidden band. 1s 2s 2p 3s 3d 3p

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Materials Characterization Principles and Techniques

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X-ray Diffraction (XRD) n = integer number for constructive interference. = the wavelength of the incoming and outgoing X-ray. = the diffraction angle. Great technique for identifying crystal structures

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Seebeck Effect

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[a] Before Thermal Excitation [b] After Thermal Excitation Seebeck coefficient, S = - V/ T. Typical values in the order of V/( o ). It can give an easy carrier type determination for semiconductor substrates.

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How Do We Measure Seebeck Effect

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Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) Fig. 1 Scanning Electron MicroscopeFig. 2 Scanning Electron Microscope

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Giant Seebeck Coefficient Observed in Manganese Oxide Nanostructures

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Why Manganese Oxide Nanoparticles? FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012).

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Einstein Prediction For Lowest Thermal Conductivity

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XRD of our as-received MnO 2 powder Fig. 1 X-ray diffraction of as-received MnO 2 powderFig. 2 NIST MnO 2 standard

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Crystal Structure of our Manganese Oxide powder Rutile structure. Gray atoms are Mn. Red Atoms are O. http://en.wikipedia.org/wiki/File:Rutile-unit-cell-3D-balls.png

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SEM and TEM images of MnO 2 powder 1. FangFang Song, Liming Wu and S Liang, Nanotechnology 23 (2012) 085401 (4pp) Fig. 1 Scanning electron microscope image of MnO 2 [ref. 1] Fig. 2 Transmission electron microscopy image of MnO 2.

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Figure of Merit and Harman Transient Method Thermoelectric materials are characterized by the figure of merit ZT. Sigma ( ) electrical conductivity. S Seebeck coefficient. Kappa (k) thermal conductivity. ZT = V DC /V AC - 1

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Preliminary Results ZT vs. MnO 2 particle stacking density

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Preliminary Results Seebeck vs. MnO 2 particle stacking density

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Preliminary Results Thermal conductivity vs. MnO 2 particle stacking density

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Conclusions MnO 2 nanoparicles are promising for creating devices: Seebeck coefficient can be improved, conductivity can be improved, and they exhibit very low thermal conductivity. MnO 2

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THANK YOU

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Giant Seebeck Coefficient Thermoelectric Device of MnO 2 powder

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FangFang Song, Liming Wu and S Liang, Nanotechnology 23, 085401 (2012). How does the Figure of Merit behave as a function of temperature, particule size, and particule density? By applying the transient Harman method we can find an answer to all these questions. Important Problems to be Studied

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Doping Semiconductors INTRINSIC do not conduct electricity because electrons are tightly bonded to the nucleus. N-type doping with Phosphorous atoms introduce an extra electron in the conduction band. P-type doping with Boron atoms introduce an extra hole in the conduction band.